Applied
voltage
(Impressed
Current) |
Since all corrosion circuits exhibit a potential (i.e. voltage) difference between the corroding anode and the reducing cathode, one technique to minimize this potential is to place a battery in the circuit that attempts to reverse the voltage difference. In effect, the battery pushes electrons back into the anode, whose original intent was to donote electrons to the cathode and corrode.
An application that uses this technique to reduce corrosion is in the protection of underground pipelines. Typically, by saving the pipeline metal the corrosion is shifted to one of the terminals of the battery. However, this is not so bad since a localized battery can be maintained easier than miles of pipeline.
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Sacrificial
anode |
In order to save the anodic corrosion-intent metal in a corrosion circuit, another metal than is more anodic (i.e. lower on the Electrochemical Series) can be brought into contact with the original metal. Now, a composition cell is set up between the original metal and the new more active metal, promoting the corrosion of the new metal. In effect, the new metal acts as a sacfrificial anode, since any corrosion that was to occur will shift to this metal.
Applications include the protection of ship hulls and water tanks (e.g. adding a block of zinc to a steel hull). Also included are parts made from galvanized steel, where a steel component is plated with zinc. If the coating suffers a scratch, the zinc will corrode more readily than the underlying steel, thereby protecting the steel.
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